The present invention relates to plasticized poly(imide-siloxane)s which have low smoke values. More particularly, the present invention relates to the employment of hydrated zinc borate and an organic plasticizer with poly(imide-siloxane)s to provide adherent wire coating compositions which satisfy the low smoke requirements of plenum wire.
Prior to the present invention, considerable effort was expended by the wire coating industry for screening various materials for the manufacture of plenum wire. As known by those skilled in the art, plenum wire is cable approved by Underwriters Laboratories for installation in central air handling systems. A principal consideration is that the insulating material must show a reduced smoke release rate when burned.
Teflon resin is widely recognized by the wire coating industry as an outstanding material with respect to reduced smoke generation. However, as reported by Y. C. Alarie et al., Fire Materials, 1984, 8, 52 and Y. C. Alarie et al., Fire Science Technology, 1982, 1, Teflon resin can generate extremely toxic vapors when exposed to flame Ultem.RTM. resin manufactured by GE also has gained attention for its low smoke properties. Even though it has many desirable characteristics, Ultem resin is not used extensively as a plenum wire coating insulator as it does not satisfy the processing characteristics or mechanical properties for directly coating the wire by standard techniques. An improvement in polyetherimide processing is shown by Cella et al., U.S. Pat. No. 4,690,997, incorporated herein by reference, which incorporates through amine-anhydride condensation, a diorganosiloxane block into the resulting polyetherimide. Although Cella et al. has demonstrated that particular silicon-polyimides can be satisfactorily extruded onto wire to produce a flame retardant wire coating, these materials have not satisfied the stringent low smoke requirements of the plenum wire coating industry. Cella et al., U.S. Pat. No. 4,833,190 further enhanced the low smoke properties of the silicon-polyimides of U.S. Pat. No. 4,690,997 by use of certain metal borates, such as zinc borate. Even though substantial smoke reduction has been achieved using these blends of silicon-polyimide and hydrated zinc borate, these blends, nevertheless, showed a Total Smoke/M.sup.2 value or "TS" value exceeding 500 after a 5 minute burn, in accordance with the Ohio State University Test, or "OSU" Test. In order to satisfy the stringent requirements of the plenum wire industry, it is desirable to have a TS value of 100 or less. The Ohio State University Smoke and Heat Test (ASTM-E96) is defined as follows:
Polymer plaques (6".times.6".times.1/16") are mounted in the horizontal mode of the test chamber. The environmental test chamber contains a constant flow of air throughout the test. Combustion is initiated by a non-piloted ignition, piloted ignition of evolved gases, or by point ignition of the surface. If ignited by piloted ignition, the end of the burner tubing is 10 mm above and perpendicular to the exposed horizontal surface of the specimen. The changes in optical density of the gas leaving the chamber are monitored and from this data, smoke release rates are determined. The results for total smoke evolved and smoke release rates are reported in smoke/squared meter of sample and smoke/min/squared meter respectively. The "SMOKE" unit is defined by the concentration of smoke particulates in a cubic meter of air that reduces the percent transmission of light through a 1 meter path to 10% as measured by a calibrated photometer.
Calculations: SMOKE Release Rate=D/kLA(V.sub.o /t)
where:
K absorption coefficient=1.0 squared meter/smoke PA1 D optical density (absorbance)=log (100%T) PA1 L light path=0.134 m (stack width) PA1 A exposed surface area of specimen, squared meter PA1 V.sub.o /t flow rate of air leaving apparatus, cubic meter/min=V.sub.i /t x T.sub.o /T.sub.i PA1 V.sub.i /t flow rate of air entering apparatus, cubic meter/min PA1 T.sub.i,T.sub.o absolute temperature of air in and out of apparatus respectively.